Multistage vapor-liquid contact process for producing fresh water from salt water

ABSTRACT

The invention is an improvement in distillation process for producing fresh water from salt water. The primary heat exchange is obtained by vaporizing a water immiscible fluid (such as a hydrocarbon) which boils at a temperature lower than water which is then recondensed by direct contact. Substantial simplification of plant, maintenance, etc. results. The invention is applicable generally to aqueous solutions of nonvolatile solutes, and to nonaqueous solutions of nonvolatile solutes.

I United States Patent [151 3,640,850 Smith, Jr. 7 Feb. 8, 1972 [54]MULTISTAGE VAPOR-LIQUID 3,219,554 11/1965 Woodward ..202/ 173 CONTACTPROCESS FOR PRODUCING 3,232,847 2/ 1966 FRESH WATER FROM SALT WATER3,298,932 1/1967 3,410,339 11/1968 [72] Inventor: Calvin S. Smith, Jr.,El Cerrito, Calif. 3,446,712 5/1969 731 Assignees: Harrison w. Sigworth,Orinda; Thomas N. 5132.

Finical, Jr., San Carlos, Calif. part interest to each PrimaryExaminerNorman Yudkoff [22] Filed: Mar. 16, 1970 Assistant ExaminerJ.Sofer [211 App! 19 592 Attorney-Gregg&l-iendricson [57] ABSTRACT [52] g"The invention is an improvement in distillation process for 159/2roducing fresh water from salt water. The primary heat [51] Int Cl B01 d3 B01 d 3/02 exchange is obtained by vaporizing a water immiscible fluid[58] Fieid Ill 21 24 100 (such as a hydrocarbon) which boils at atemperature lower d i than water which is then recondensed by directcontact. Substantial simplification of plant, maintenance, etc. results.The [56] References Cited invention is applicable generally to aqueoussolutions of nonvolatile solutes, and to nonaqueous solutions ofnonvolatile UNITED STATES PATENTS solutes.

2,749,094 6/1956 Lewis et a1 ..165/1 12 Claims, 9 Drawing FiguresTAEATJD o 88 IA ra zd/Vl Z 5W We .93 94 5.3 J 445 .5

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6/ 62 [ND 5 5w a: COLD d FWII/ 7/b 74 0:740 N 714/ 75- 6; I410MULTISTAGE VAPOR-LIQUID CONTACT PROCESS FOR PRODUCING FRESH WATER FROMSALT WATER BACKGROUND OF THE INVENTION 1 Field of the invention Thisinvention is an improved process for the production of fresh water fromsalt water. (Other applications are discussed below). More particularly,this invention is an improvement on the long known distillation processfor production of fresh water from salt water, said improvementeliminating a large proportion of the large heat exchange equipmentrequirements of the conventional process. Said improvements lead to anintegrated process for the production of fresh water from salt waterwhich is simple in concept and in equipment requirements; is susceptibleto large scale plants; is low cost to operate; and does not require theconstruction of other large facilities such as a nuclear power plant toobtain low cost steam for economic operation.

2. Prior Art In general, the leading processes for production of freshwater from salt water include the following:

1. Distillation, with which this invention is concerned.

2. Crystallization, either by freezing or by hydration.

3. Reverse osmosis.

4. Solvent extraction.

Of these processes, distillation seems to be the best with respect tocost, and quality of product, and has been the process selected by theUnited States Office of Saline Water for the design of the largeprototype plants. There are three main types of plants for production ofwater by distillation, as follows:

1. Vapor Compression Distillation In this type of plant, salt water isheated by exchange with the products and usually makeup steam to theprocess temperature. The salt water is then boiled, and the steam isthen compressed (usually with a mechanical compressor) and is then usedto boil the salt water in out-of-contact heat exchange, the steam beingcondensed as fresh water in out-ofcontact heat exchange. This type ofplant is (compared with some of the other plants) simple, and isefficient, and in small plants has had widespread application on shipsas 'the Kleinschmidt still.

2. Multiple Effect or Long Tube Vertical (LTV) Process Salt water ispretreated to reduce heat exchanger fouling and corrosion and is heatedto about 250 F. with low pressure steam from a topping turbine in apower plant in out-of-contact heat exchange. The steam recovered byboiling salt water in the first stage is then used at a slightly lowerpressure to boil salt water in the second stage. Steam from the secondstage is used at slightly lower pressure and temperature to boil waterin the third stage, etc. for five to 12 stages.

To reduce pressure drop, the salt water is pumped to the top of eachstage where it falls on the inside wall of the exchanger tube as a filmin concurrent flow with the vapor. Hence the name long tube vertical. Onthe outside wall of the exchange tube, steam is condensed as a productfresh water.

3. Multistage Flash vaporization The multiple flash process (MSF) iscomplex but nevertheless appears to be the leading distillation process.Fresh salt water and recycle brine are preheated in out-of-contact heatexchange in successive stages by vapor from flashing salt water to about250 F. and then by steam to about 260 F. The hot salt water is thenflashed in 50 to 100 successive stages each at lower pressure, the steambeing condensed as product by the beforementioned salt water beingheated in tubes.

In addition to requiring expensive heat exchange, a disadvantage of thisdesign is that failure of even one of the many tubes can shut down theplant. As salt water in the tubes is at higher pressure than thecondensing fresh water, leakage is always salt water to fresh, and evenminor leakage cannot be tolerated.

Certain problems are common to all processes which rely upondistillation. As will be shown the magnitude of these problems isgreatly reduced by the present invention.

A major problem whose magnitude is greatly reduced by the presentinvention is the cost of heat transfer. About 10,000 Btu. of heat mustbe transferred for each gallon of fresh water produced by thedistillation process. Prior processes generally use tubular heatexchangers constructed of metal tubes. The large heat transfer duty ofthese tubes requires a very large surface area, for example in excess of5,000,000 square feet for a 50,000,000 gallon a day plant. Surface areasof such magnitude are expensive to install in the first instance and dueto the corrosive and scale forming properties of sea water and brine,especially at the elevated temperatures involved, give rise to majorproblems and require costly pretreatment to alleviate corrosion andscale forming. The present invention greatly reduces surface areassubject to corrosion and scale formingl Another problem is tube failurewhich is bound to occur due to one or more factors such as faultymaterial, faulty tube construction, faulty installation, stress, strainand water and tear during use, the corrosive action of sea water andelectric, chemical and biochemical action. Tubes having very thin wallsare employed to aid heat transfer, but such tubes are frail and areespecially prone to failure. In a plant employing a large number of heatexchange tubes if a single tube fails it may shut down the entire plantor unit and the likelihood of failure of a single tube increasesexponentially with the number of tubes. The present invention makes itpossible to eliminate tubes completely in the major heat transfer areasand to greatly reduce the probability of failure that will shut down aplant or a unit.

Yet another problem with existing distillation methods is the problem ofproviding a cheap source of steam which matches the requirements of thedistillation plant and is economical to use. Lower value steam frompower plants such as from topping turbines in steam electric plants isproposed as an economical source of steam. Construction of a big waterplant requires simultaneous construction of a big power plant. Moreover,the low pressure steam available at an electric plant is greatlyvariable from hour to hour as a power plant commonly operates at about50 percent load factor. This means that the output of the distillationplant must vary with the output of the steam electric plant and thatoperation of the distillation plant will swing the performance of thepower plant. In certain preferred embodiments of my invention steam froma power plant can be advantageously used. In a second preferredembodiment of my invention, a gas turbine can be used as a prime moverto operate compressors, pumps, etc., in the distillation plant; thewaste heat of the turbine can be used for heat input directly and/or byway of a steam boiler and the gas turbine can be matched with therequirements of the distillation plant whereby the latter can beoperated uniformly at an optimum rate.

General Description of the Invention In accordance with the presentinvention heat exchange is effected by direct contact between an aqueousphase and a nonaqueous phase which is (1) lower boiling than water and(2) is immiscible with water. Preferably the nonaqueous phase has asubstantially different density than water such that it will eitherfloat on top of a body of water or will sink beneath the water and causethe water to float on top. This facilitates separation of the two phasesby gravity. Most advantageously the nonaqueous phase is substantiallylighter in density than water.

For simplicity and clarity the nonaqueous phase will be referred to asimmiscible vapor when it is in the vapor form, as immiscible liquid"when it is in the liquid form and where the context requires referenceto both phases it will be referred to as the immiscible medium.

The immiscible medium may be a pure material such as pentane (which islighter than water) or carbontetrachloride (which is heavier than water)or it may be a mixture of molecular species having a narrow boilingrange or a rather wide boiling range. Mixtures may be simple mixturessuch as pentane-hexane mixture or a more complex mixture such a mixturecontaining C C C and C-, paraffinic hydrocarbon. Examples of suitableimmiscible media are as follows:

Normal and branched chain pentanes Normal and branched chain hexanesNormal and branched chain heptanes Mixtures of any or all of the aboveMixtures of any of the above with normal or isobutane CyclohexaneBenzene Mixtures of benzene and cyclohexane Mixtures of any of the aboveparaff'mic hydrocarbons or hydrocarbon mixtures with benzene,cyclohexane or mixtures of benzene and cyclohexane Furan Amongimmiscible liquids that are more dense than water and which are suitablefor purposes of the present invention are chloroform,carbontetrachloride, dichloromonofluoromethane.

In its general aspects, the process of the present invention is asfollows:

I. Relatively cool sea water (or other aqueous solution of a nonvolatilesolute) is brought into heat exchange relation with relatively warm orhot vapor of the selected immiscible medium whereby the immiscible vaporis condensed to immiscible liquid and the water is heated by the latentheat of the immiscible vapor. As will be apparent from the detaileddescription below, the aqueous solution may be brought into directcontact with immiscible vapor, or the aqueous solution may be covered byand in direct contact with a layer of immiscible liquid which in turn isin direct contact with immiscible vapor, but in either case latent heatis recovered by direct liquid vapor contact.

2. Heated water from (I is caused to distill to provide fresh watervapor.

3. lmmiscible liquid resulting from condensation of immiscible vapor instep (I) is directly contacted with heated fresh water to vaporize theimmiscible liquid and produce immisciblevapor and to cool the freshwater. The immiscible vapor from this step is recycled to step l 4.Cooled fresh water resulting from step (3) is contacted with water vaporresulting from step (2) to condense the water vapor. A portion of theresulting fresh water is withdrawn as product and the remainder isrecycled to step (3).

At one or more appropriate places in this system a heat input isprovided, for example by conventional heat exchange using a tube-typeheat exchanger with steam as the source of heat or by condensation ofsteam in the circulating fresh water, or by the use of a compressor tocompress immiscible vapor or water vapor and raise its temperature or byusing a warm salt water stream from a power plant, or by using warm seawater near the surface in certain areas of the ocean. A combination oftwo or more such means of heat input may be employed.

The invention will now be described in more detail with reference totheaccompanying drawings in which:

FIG. 1 is a simplified diagrammatic illustration of the process of theinvention;

FIG. 2 is a similar diagrammatic illustration showing certain mechanicalfeatures;

FIG. 3 is a view in vertical cross section taken along line III-III ofFIG. 2 showing a suitable physical relationship between the salt waterdistillation zone (zone 3) and the fresh water vapor condensing zone(zone 4);

FIG. 4 is a view in vertical cross section taken along the line IV-IV ofFIG. 2 showing a suitable physical relationship between the saltwater-immiscible vapor zone (zone 1) and the fresh water-immiscibleliquid zone (zone 2);

The various zones are shown unfolded in FIG. 2 for purposes of clarityand in FIGS. 3 and 4 they are shown in actual physical relationship;

FIG. 5 is a somewhat diagrammatic illustration of yet another embodimentof the invention;

FIG. 5A is a diagrammatic view showing how saline water is pretreated,preheated, etc. This figure is to be considered in relation to FIG. 5.Lines common to both figures bear the same reference numerals.

FIG. 6 is vertical cross-sectional view taken along the line VI-VI ofFIG. 5,'showing the physical relationship of the various zones;

The various zones are shown unfolded in FIG 5 for purposes of clarityand in FIG. 6 they are shown in actual physical relationship;

FIG. 7 is a partial view of a variant of FIG. 5;

FIG. 8 is a vertical cross-sectional view taken along the linesVIII-VIII of FIG. 7;

FIG. 9 is a horizontal transverse view of the upper part of FIG. 6 takenon the line IX-IX.

The zones are shown unfolded in FIG. 7 for purposes of clarity and inFIG. 8 they are shown in actual physical relationship.

In the drawings certain fluids are identified by lettered symbols whichare identified by a legend on the same sheet of drawings as FIG. 1 andwhich are set forth below for convenience of reference:

LEGEND Symbol Meaning SW Saline water FW Fresh water IV lmmiscible vaporIL lmmiscible liquid WV Water vapor BR Brine Referring now to FIG. 1,four zones are shown which are identified as zones 1, 2, 3 and 4. Se awater enters zone I by line 10 and immiscible vapor enters by line 11.The immiscible vapor is hotter than the sea water and is condensed bydirect contact with the sea water (or with a body of immiscible liquidwhich in turn is in direct contact with sea water), thereby giving upsensible heat and latent heat to the sea water and raising thetemperature of the latter. As a result the immiscible vapor is condensedto form immiscible liquid which flows though line 12 to zone 2 whereinit is heated and revaporized by direct contact with warm fresh waterentering through line 13. This results in vaporizing the immiscibleliquid and cooling the fresh water. The immiscible vapor is recycledthrough line 11 to zone I and cooled fresh water passes to zone 4through line 14 for utilization as described below.

Heated sea water produced in zone 1 leaves by line 15 and distills inzone 3 (a zone of lower pressure) to produce vapor of fresh water whichleaves by line 16. The remaining cooled sea water or brine is recycledto zone 1 by line 17. A portion of this effluent brine is removed fromthe system by line 18 to prevent excessive accumulation of the salts andother solids in the system. The fresh water vapor produced in zone 3 isled to zone 4 by line 16 where it is directly contacted with cold freshwater produced in zone 2 and which is introduced into zone 4 by line 14.This results in condensation of water vapor and warming of the freshwater effluent from zone 2. This warm water is recycled by line 13 tozone 2 and a portion is withdrawn through line 19 as product.

It is, of course, necessary to provide a heat input. Such heat input maytake any one or more of several forms. As shown, a heat exchanger 24 maybe installed in line 15, for example a tube-type heat exchanger whereinthe sea water is passed through tubes in a jacket in countercurrentrelation to steam flowing through the jacket. Alternatively, immisciblevapor in line 11 may be compressed or water vapor in line I6 may becompressed. Alternatively, steam may be condensed in line 13.Alternatively, in areas where warm sea water is. available near thesurface and it is practical to obtain cold sea water at a depth of about1,000 feet not too distant from the coast, surface sea water may beemployed as a source of heat input. Cold sea water will be used tocondense immiscible vapor. This embodiment (use of cold sea water from adeep source and a warm surface sea water) is described below withreference to FIG. 2. Other methods of heat input may be used andcombinations of several methods may be employed.

It will be apparent that certain marked advantages are provided by thesystem of FIG. 1. Heat transfer occurs in zone 1 between immisciblevapor and sea water in zone 2 between immiscible liquid and fresh water.Such heat transfer is accomplished by direct vapor-liquid contact,therefore heat transfer surfaces such as metal tubes are unnecessary.Since it is the latent heat of condensation of the immiscible vapor(which is relatively very large) rather than sensible heat (which isrelatively quite small) which provides the bulk of the heat transfer, alarge amount of heat can be transferred without large temperaturedifferentials and without having to transfer excessively large volumesof liquid and gas.

Referring now to FIGS. 2, 3 and 4, zones 1, 2, 3 and 4 there showncorrespond to similarly numbered zones of FIG. 1. That is, in zone 1 seawater is heated by condensation of immiscible vapor; in zone 2immiscible liquid from zone 1 is vaporized by direct contact with hotfresh water; and in zone 3 sea water heated in zone I is distilled toproduce fresh water vapor which is condensed in zone 4 by contact withcold fresh water from zone 2.

Zone 1 is shown with an agitator such as a rotatable shaft having mixingblades or paddles fixed thereto at intervals. As shown in FiGS. 2 and 4,zone 1 contains a bottom layer 21 of sea water, a layer of immiscibleliquid 22 overlying the sea water and an immiscible vapor space 23 abovethe immiscible liquid. The agitator 20 may be confined to the interfacebetween the liquid phases and closely adjacent areas. Alternatively, theimmiscible liquid layer may be absent or very thin and the agitator 20may be located so as to intermix sea water with immiscible vapor.Factors which govern this choice are discussed below. The agitator 20 isrotated by any suitable means (not shown) to promote direct contactbetween the two liquid phases or between immiscible liquid andimmiscible vapor and to bring about efficient, rapid heat exchangebetween the two phases such that the immiscible vapor is condensed andgives up its sensible and latent heat to the sea water thereby raisingthe temperature of the sea water. Cold sea water enters zone 1 throughline 24. The sea water passes through a packed section 25 where it iscontacted with immiscible vapor vented from zone 1 and is stripped ofdissolved gases. The mixture of immiscible vapor and gases is ventedthrough line 26 and valve 27 and immiscible liquid is recovered bysuitable means and is restored to the system. Immiscible vapor entersthe opposite end of zone 1 through line 28 and flows from left to rightwhereas sea water flows from right to left as viewed in FIG. 2.Therefore, countercurrent flow of sea water and immiscible vapor areprovided. Intimacy of contact and efficiency of heat transfer arepromoted by agitator 20. Warm salt water leaves zone 1 through line 29and is treated as described hereinbelow.

At the right-hand end of zone I immiscible liquid resulting fromcondensation of immiscible vapor by cold sea water is withdrawn throughline 30 and is introduced into one end of zone 2. Warm fresh water isintroduced into the other end of zone 2 through line 31. Zone 2 isequipped with a mixing device 32 similar to the mixing device 20 inzone 1. Flow of immiscible liquid and fresh water are countercurrent.Heat exchange between the immiscible liquid and fresh water is effectedin zone 2, thereby vaporizing immiscible liquid, producing immisciblevapor and cooling the fresh water.

As in the case of zone 1, zone 2 contains a bottom layer 33 of liquidaqueous phase (in this case fresh water) an overlying layer 34 ofimmiscible liquid and an immiscible vapor space 35 above the immiscibleliquid. As in zone 1, the agitator 32 is confined to the interfacebetween the liquid phases 33 and 34 and closely adjacent areas.Alternatively, the immiscible liquid layer 34 may be very thin and theagitator 32 may be located so as to intermix water directly with thevapor, but it is preferred to maintain a sufficiently deep layer ofimmiscible liquid to prevent this and to minimize evaporation of waterin zone 2.

The immiscible vapor produced in zone 2 is recycled throughline 28 tozone 1. Inasmuch as it is not desirable to evaporate all of theimmiscible liquid in zone 2 the unevaporated residue of immiscible istransferred through line 28a to zone 1.

Warm sea water having zone 1 through line 29 is introduced into zone 3.Makeup sea water, which is suitably warmed e.g., because it is takenfrom the surface of the sea in an area where the surface temperature ishigh or because there has been a heat input such as from power plantcondensers is also introduced into zone 3 through line 36.

Zones 3 and 4 are shown as consisting of n stages. Typically n may havea value of one to I00. Referring to FIG. 3, in which one of the stagesof these zones is shown in elevation, a continuous barrier 39 isprovided between the stages to prevent mixing of the liquids in the twostages but to allow water vapor (and some immiscible vapor) to passbetween. Each stage is separated from adjacent stages by a barrier 40(see FIG. 2) which extends from the top of the two zones downwardly butnot to the bottom. A contacting device 41, which may be the same as orsimilar to the mixing device 20 in zone 1, is provided in zone 4. As seawater moves in zone 3 from left to right it distills and water vapor(and some immiscible vapor) so produced in each stage passes to and iscondensed in the corresponding stage of zone 4. Such condensation isaided by contacting device 41. The condensing medium is cold fresh waterthat leaves zone 2 through line 42. Sea water that has been heated inzone 1 by the sensible and latent heat of immiscible vapor is introducedinto an appropriate stage of zone 3 wherein the temperature of the seawater flowing in line 29 matches the temperature of sea water flowing inzone 3. Brine is removed from the system through line 43, part of whichmay be recycled to zone 1 through line 24. A vacuum is applied to zone 4through line 44 and the vapors (uncondensed water vapor and immisciblevapor) are treated in a recovery system to recover and recycle theimmiscible liquid. Fresh water is removed from zone 4 and recycled tozone 2 through line 31. A portion of the fresh water is removed asproduct through line 46. Baffles 47 are provided in zone 3 to promoteturbulence and assist vaporization of water.

Referring now to FIGS. 5 and 6, zones 1, 2, 3 and 4 correspond tosimilarly numbered zones of the preceding figures, i.e., in zone 1 thelatent and sensible heat of immiscible vapor is employed to heat seawater; in zone 2 the sensible heat of fresh water is employed towaporize immiscible liquid; in zone 3 sea water is distilled and in zone4 the vapor of fresh water produced in zone 3 is condensed by contactwith liquid fresh water. A more elaborate and flexible system isprovided than in the apparatus of the preceding figures. Brieflydescribing the structure of FIGS. 5 and 6; Zone 1 is equipped with amixing device 50 to promote contact of immiscible liquid and sea waterand with a second mixing device 51 to promote contact of immisciblevapor with immiscible liquid. Zone 2 is provided with a mixing device 52to promote contact between fresh water and immiscible liquid. Zone 3 isdivided into stages (indicated as a to k but the number of stages mayvary from plant to plant) by baffle pairs 53, 54 (see FIG. 5).Continuous spaced baffles 55, 56 separate zone 3 from zone 4 (see FIG.6). Effluent heated salt water from zone 1 travels up a leg or standpipe 57 with the aid of a low pressure pump or impeller 58. Sea waterthat has been partly distilled in zone 3 is recycled to zone 1 throughleg or stand pipe 59. Zone 4 is divided in corresponding stages athrough k by spaced baffle pairs 60, 61. The spaced baffles 55, 56 (seeFIG. 6) that separate zone 3 from zone 4 provide channels 62 tocommunicate the respective stages and permit passage of water vapor (andsome immiscible vapor) from zone 3 to zone 4. Intimate contact of suchvapor and liquid water is obtained by vapor distributors such as sievetrays or bubble cap trays 62a.

Fresh water that has been cooled by evaporation of immiscible liquid inzone 2 rises through leg or stand pipe 63 with the aid of a low pressurepump or impeller 64 to zone 4 and serves to condense water vapor in zone4. The resulting fresh water is recycled to zone 2 through leg or standpipe 65. A portion of this circulating fresh water is withdrawn, asproduct at 66.

A heat input is, of course, required. This heat liquid may take any oneor more of several forms, several of which are described below withreference to FIG. 5.

The system as thus far described as certain advantages. For example, inzone 1 sea water flows countercurrently to immiscible liquid. Heatexchange between these phases is promoted by mixing device 50. The layerof immiscible liquid acts as a seal and applies a hydrostatic pressurewhich prevents substantial contact of the liquid water phase with thelargely immiscible vapor phase, therefore minimizes the partial pressureof water vapor in the largely immiscible vapor phase by impeding WV flowinto the IV phase. Mixing device 51 promotes heat exchange contactbetween the immiscible vapor and immiscible liquid phases. Theimmiscible liquid entering zone I through line 85 and immiscible vaporentering through line 75a contain a maximum of high boiling componentsat a temperature greater than the temperature of the SW at that pointand therefore transfers heat to the SW. Preferably, the system of FIG. 5employs an immiscible liquid having a fairly large boiling range, e.g.,boiling between about 80 and 170 F. and is a mixture such as gasolinehaving a rather uniform, continuous distribution of low, high andintermediate boiling constituents such that the boiling point curveapproximates a straight line. Therefore, there is a minimum temperaturedifference at any point between immiscible vapor phase and immiscibleliquid phase and between immiscible liquid phase and liquid water phase.This condition provides efficiency in that the change in entropy isminimized.

Similarly in zones 3 and 4 (although they are physically separated) flowof fresh water entering in cold condition at 63 and sea water enteringin hot condition at 57 is countercurrent and the temperature differencefrom each stage in zone 3 to the corresponding stage of zone 4 is smalle.g., about 1 to 4 F. The large open channels 62, see FIGS. 6 and 9,provide minimum pressure drop from zone 3 to zone 4. Direct contact witha bubbling device permits design for low temperature differences.Therefore, change in entropy between zones 3 and 4 is minimized.

Moreover, advantage is taken of hydrostatic pressure differcntials suchthat energy for pumping is kept to a minimum. As heated sea waterascends through leg 57 its hydrostatic head diminishes. Therefore, whenit reaches stage a of zone 3 it will flash and yield an incrementofwater vapor and will cool slightly. As the sea water proceeds up theladder of zone 3 at each succeeding stage its hydrostatic headdiminishes somewhat and a further increment of water vaporizes and theresidual unvaporized water is reduced slightly in temperature. Thelowest hydrostatic head is exerted at the upper end of the ladder, whichcorresponds with the lowest boiling temperature. As the concentrated seawater or brine descends through leg 59 its hydrostatic head increasesthereby restoring the head lost in leg 57 and in the ladder of zone 3.Therefore the head required at pump 58 is principally limited tofrictional losses. Similarly on the fresh water side of the apparatus,loss of hydrostatic pressure in leg 63 is restored to a major extent asthe fresh water goes down the ladder and through leg 65.

Certain advantages adjuncts of the system just described will now bedescribed.

Sea water enters the system through line 69. in conventionaldesalination plants in which there is a large amount of heat exchange itis desirable to heat the incoming sea water to minimize scale formation.Because of the much lesser amount of heat exchange and heat exchangesurfaces in the present system, pretreatment of the sea water is not asimportant. Nevertheless it may be employed and is illustrated in FIG.5A. Thus sulfuric acid in suitable amounts, e.g., 60 to 120 parts permillion of sea water, is introduced by line 70. Suitable mixing means(not shown) may be used to mix the acid uniformly with the sea water. Aportion of the treated cold sea water is withdrawn through line 71 forvarious purposes as described below. The balance (and the major portion)of the treated cold sea water is then passed through a heat exchangersystem 72 wherein it is in heat exchange relation with reject brine thathas been withdrawn from the system through line 72a and with productwater that leaves the system through line 66. This heat exchange systemprovides several advantages as follows: (1) it transfers waste heat tothe incoming sea water thereby conserving the heat input to the system;(2) it cools the hot brine which is desirable because the dumping of hotbrine into the ocean or a stream is undesirable and may kill fish orother marine or plant life; and (3) the hot product water is cooled forstorage, transfer and/or use. All these objectives are accomplished bythe heat exchange systemjust described and illustrated. The heatexchanger may be of any desired type and if the brine is hotter than theproduct water or vice versa the heat exchanger may be arrangedaccordingly so that there is countercurrent flow of incoming cold seawater and reject brine and product water in a thermal sense to preheatthe SW.

The treated and heated sea water proceeds then through line 69 to acontacting zone 73, such as a packed column and proceedscountercurrently to a stream of gas and vapor from zone 1 containinguncondensed immiscible vapor and carbon dioxide, nitrogen and oxygen.This strips the sea water of its dissolved gases and condensesimmiscible vapor vented from zone 1. The sea water (and immiscibleliquid resulting from condensation of immiscible vapor) then pass intozone 1 for countercurrent flow with respect to and heating by condensingimmiscible vapor as described above. Effluent vapor and gas from zone 73passes through a zone 74 where it is con tacted by a countercurrentstream of cold sea water conducted to this zone by a branch of line 71identified by the reference numeral 71a. This cools the effluentvapor-gas mixture which is vented through a pressure reducing valve 75.The vented gas may be subjected ,to an additional vapor recovery step ifdesired, e. g., by refrigeration or absorption by a solid or by a liquidsolvent to recover any uncondensed immiscible medium for recycling tothe system.

lmmiscible vapor passing from zone 2 to zone 1 through line 75a may becompressed by compressor 76 thereby providing a heat input to thesystem. Another source of heat input, which may be an alternative to oran addition to the compressor 76, is a heat exchanger 77 through whichcondensing steam may be passed and through which a portion of thecirculating sea water in leg 57 may be passed by way of a line 78 andpump 79.

lmmiscible liquid from zone 2 is pumped through line by pump 86 to zoneI for ease of control and assurance of similar liquid compositions inzone 1 and zone 2. The proportion of immiscible liquid thus transferredis, however, small compared to that which is vaporized.

A portion of the treated cold salt water removed from line 69 by way ofline 71 is passed through a branch line 71b to a heat exchanger 87 forthe primary purpose of condensing irnmiscible vapor that is vented fromthe upper end of leg 63 or from stage k of zone 4. This condenses thevapor into immiscible liquid which is returned by line 88 to zone 2.Unconden sed gas and vapor are removed by line 89 to a vacuum system. Atthe lower end of the ladder of zone 4, in zone 0 thereof, in

which the hottest vapor from zone 3 is condensing, immiscible vapor andwater vapor are vented through line 90 and a pressure reducing valve 91and are passed through a heat exchanger 92 where they are cooled by aportion of the cold treated sea water taken from line 71 by line 710.lmmiscible liquid resulting from condensation of immiscible vapor inheat exchanger 92 is separated from gases in vessel 93. The gases arevented through line 94 and the condensate of immiscible liquid isreturned to the system through line 95. Sea water diverted through lines71b and 71c is returned to line 69 and the rest of the system. The heatexchange-condensation systems shown at 8789 and 92-95 serve also toseparate the small amount e.g., about 20 to I00 parts per million) ofimmiscible liquid dissolved and entrained in the water in ducts 57 and63.

As stated above, the immiscible liquid may be a simple molecular speciessuch as pentane, or a mixture of molecular species having a very narrowboiling range, or it may be a mixture having a wide boiling range and afairly uniform distribution of molecular species such that the boilingpoint curve approximates a straight line. Referring to FIG. 5, animportant advantage of an immiscible liquid such as gasoline having awide boiling range and a uniform distribution of molecular species isthat the temperature from inlet line 59 to outlet line 57 of zone 1 islarge, and there is a larger yield of distilled water per pass throughzone 3 which reduces the size of certain equipment, particularly pump58, and the volume requirements for phase separation. The sameadvantages also occur in zone 2 and zone 4. A disadvantage of a wideboiling immiscible liquid is that more water vapor is vaporized alongwith the immiscible liquid in zones 1 and 2 if water is present at theimmiscible liquid-vapor interface. I have found, however, thatvaporization of water can be suppressed by operating with a substantiallayer of immiscible liquid in zones 1 and 2, and designing mixers 50 and52 carefully to provide good agitation of the water and immiscibleliquid phases for heat transfer, but to avoid swirling water to begas-liquid interface. Use of a wide boiling range immiscible liquid isparticularly important in designs according to FIG. in which compressor76 is to be minimized or eliminated, because thermal efficiency ispromoted significantly by as wide a spread as possible between the inletand outlet temperatures of each of the zones. This preferred designwould have application with low pressure, low cost steam from a powerplant.

With compressor 76 maximized in plants that have no other externalenergy source, and particularly with gas turbine energized plants, theadvantages of a wide boiling immiscible liquid are less. If the maximumboiling point of the immiscible liquid is 60 to 80 F. less than that ofwater, the amount of water that is vaporized in zone 2 with theimmiscible liquid is tolerable. In such a system a thin layer ofimmiscible liquid can be maintained in zones 1 and 2 and the mixing mayoccur at or near the interface of the liquid and vapor phases. Animmiscible liquid with a to 40 F. boiling range results in a goodcompromise on circulation rate and thermal efficiency, and results ingood stability of operation and is particularly attractive in smallplants where the complexity of numerous stages in zones 3 and 4 isundesirable. By use of a very narrow boiling immiscible liquid or a purespecies the number of stages in zones 3 and 4 can be reduced to l to 4,and bubble trays may be substituted for the agitators in zone 1, whichrepresents a further simplification.

Heat input to the system has been described in various ways, such ascompression of immiscible vapor by compressor 76 and indirect heatexchange in heat exchanger 77. It is also possible to provide a heatinput in other ways alternative to or in addition to the heat inputsdescribed. Thus, the heavy ends of immiscible liquid pumped by pump 86through line 85 from zone 2 to zone 1 may be boiled in a suitable still(not shown) thereby providing a heat input; or the water descendingthrough pipe 65 may be heated by any suitable means (not shown); or heatexchanger 87 may be replaced by a compressor and the compressed watervapor returned to the system by condensing it in stage a of zone 4.

Yet another means of heat input is as follows: Stage k of zone 3 andstage a of zone 4 are modified to eliminate the channels 62 connectingthem to corresponding stages of the other zone; water vapor from stage kof zone 3 is compressed by a suitable compressor; the compressed watervapor is introduced into stage a of zone 4, and steam is injected intothis compressed water vapor, which is then returned to the circulatingfresh water in the fresh water side (zones 2 and 4 and pipes 63 and 65)of the system. This would eliminate the need for heat exchanger 77. Themakeup energy source for the process of FIGS. 5 and 6 can vary dependingupon local availability of fuels. In areas where natural gas isavailable, the

gas turbine is an especially economical source of energy. The gasturbine is the prime mover for compressor 76. The hot exhaust gas fromthe gas turbine is passed througha waste heat boiler for recovery ofhigh pressure steam. The high pressure stem is used in topping turbinesto drive the pumps and agitators in the process and provide additionalhorsepower on the compressor 76. The exhaust steam from the toppingturbines is used as makeup heat on exchanger 77. In this design,compressor 76 would be large, and the heat input to exchanger 77relatively small. lmmiscible liquid may be either narrow boiling or wideboiling depending upon the engineering choice of plant simplicity orreduced circulation.

If the energy source is residual fuel oil or coal or nuclear fuel whichcannot be advantageously utilized in a gas turbine, high pressure steamis raised in a conventional boiler. If the steam plant is for theexclusive use of the desalination plant, compressor 76, and pumps andagitators are driven with steam topping turbines, with exhaust steambeing used in exchanger 77. If the desire is to run the steam plant forgeneration of electricity by steam topping turbines, compressors 76 canbe entirely dispensed with, but the need for steam in exhanger 77increases. A wide boiling immiscible liquid is important to reducedenergy requirements.

To those skilled in the art, it will be apparent that a great manydifferent designs are available within the spirit of this invention, andthat the optimum for any one design will vary depending upon numerouslocal considerations such as the source of fuel, and desire for electricpower.

In FIG. 7, a modification to zones 1 and 2 is shown to illustrate designchanges if the compressor 76 is absent. In this design each of zones 1and 2 is divided into 1 to n corresponding stages by vertical baffles 97from the ceiling and dipping into the immiscible liquid but not to theinterface between immiscible liquid and water, Spaced continuous bafflesare provided at 98 and 99 (FIG. 8) which provide a vapor passage 100between each stage of zone 2 and the corresponding stage of zone ll.lmmiscible vapor flows through each such passage and through agas-liquid contactors 101 in the respective stage of zone I. Thecontactors 101 may be sieve trays, or bubblers of various designs asshown in FIG. 8. By this means good point-to-point correspondence ofimmiscible liquid-vapor composition can be obtained enhancingcondensation. This open design reduces pressure drops, and hencetemperature differences and hence efficiency is increased. Vaporvelocities are decreased and hence less entrainment and a purer productresult. Without the compressor, minimum circulation is more important. Awider boiling range immiscible liquid is required. A wider boiling rangeimplies that part of the immiscible liquid will boil nearer water. Thisin turn increases the need to operate with the water phase substantiallyburied under the immiscible liquid phase to suppress vaporization ofwater. Thus in FIG. 7 agitators 52 and 50 are shown at a deep interfaceand will be designed to mix the water and immiscible liquid, with phaseseparation before the immiscible liquid is circulated to the gas-liquidinterface.

In another embodiment of my invention the heat exchange involved in heatexchanger 72 of FIG. 5 is greatly reduced or entirely avoided, thetreatment of sea water with acid as shown at 70 is greatly reduced orentirely avoided and additional brine is converted to fresh water. Thisembodiment employs the joint operation of the system of FIG. 2 and thesystem of FIG. 5 (hereinafter referred to as FIG. 2 and FIG. 5). Thisjoint operation will now be described with reference to FIGS. 2 and 5and associated figures. Lines interconnecting the two systems are shownin the drawings.

Sea water is taken from the sea (with preliminary filtration or settlingif necessary to separate coarse solid impurities) is introduced intozone 1 of FIG. 2 and is caused to flow countercurrently, as describedabove, to the flow of immiscible vapor and condensate thereof, suchimmiscible vapor being preferably that of a wide boiling range (e.g., 50to F.) immiscible liquid such as a hydrocarbon mixture. The effluent,heated sea water is not, however, passed through line 29 to zone 3 ofFIG. 2 but passes instead through line 29a, thence to inlet line 69 ofFIG. but without passing through a heat exchanger 72 and withoutintroduction of acid through line 70. That is the preheated sea waterfrom zone I of FIG. 2 is introduced directly into contacting zone 73 ofFIG. 5. (If some heat input by conventional methods and/or if sometreatment with acid is desired, the preheated sea water from zone I toFIG. 2 may be passed through a heat exchanger similar to but muchsmaller than that at 72 in FIG. 5A and/or a quantity of acid, but muchless than that introduced at 70 in FIG. 5A may be added and mixed in).

The two systems (FIG. 2 and FIG. 5) are operated as described above butwith the following differences: The heat input to FIG. 5, as stated, ispreheated sea water from FIG. 2. The input to zone 3 of FIG. 2 is thehot brine withdrawn at 72a from FIG. 5. Hot fresh water withdrawn fromFIG. 5 at 66 is introduced into FIG. 2 through line 31. Product freshwater is withdrawn with FIG. 2 through line 46 as described above.

The invention has been described in detail with reference to immiscibleliquids that are lighter than water. The same principles apply withreference to immiscible liquids that are heavier than water except thataccount must be taken of the fact that the aqueous and immiscible liquidlayers are reversed. Referring to FIG. 2, immiscible vapor (ofimmiscible liquid heavier than water) enters zone I through line 28 andis condensed therein and sinks to the bottom. Meanwhile seal water movescountercurrently but as an upper layer. The two liquid phases and thevapor phase are mixed by agitator 20. Layers 21 and 22 are reversed. Inzone 2, immiscible liquid, removed from zone 1 through line 30, whoseinlet is appropriately relocated, enters the left end of zone 2 (throughline 30, connected lower than indicated in FIG. 2) and proceeds to theright as the bottom phase countercurrently to heated fresh water movingto the left. The two liquid phases are mixed by agitator 32. Immisciblevapor passes upwardly into the vapor zone of zone 2.

It will be noted that certain immiscible liquids indicated as suitablefor purposes of the present invention are quite low boiling. Forexample, normal pentane boils at 97 F. These low boiling immiscibleliquids are, nevertheless, useful in the practice of the inventioneither per se or as the low boiling components of a wide boilingmixture. Thus, pressure is maintained in the sea water preheating zoneI. For example in the system of FIG. 5 the pressure in zone 1 may be 65p.s.i. and the pressure may diminish to a few pounds per square inch atthe top of the ladder of zone 3. The long legs 57 and 59 act as pressureseals between the high and low pressure zones and, as described, thehydrostatic pressure in leg 59 restores most of the pressure that islost in going up leg 57 and the ladder.

The descriptions above with reference to FIGS. 2 and 5 relates to theuse of a constant pressure in zone I and a constant pressure in zone 2.It is possible however to modify these zones to employ a varyingpressure. For example, zones 11 and 2 can be modified or include stagesas in zone 3 or zone 4 of FIG. 5 (with or without vapor communicationbetween individual stages of zone I and zone 2). In such modifiedconstruction, a narrow boiling, an intermediate boiling or a wideboiling range immiscible liquid may be employed.

It will therefore be apparent that a novel and very useful method ofdesalinating sea water is provided. The process is applicable not onlyto sea water but also to other salt water sources such as salt waterbrines, brackish inland waters, etc. The method may be used in processeswhere the main product or a byproduct is the solute, e.g., salt, otherminerals, etc. The method is also applicable to the concentration byevaporation of weak liquors such as waste sulfate liquors in the paperindustry, liquors of aluminum sulfate, ammonium sulfate, etc.

I claim:

l. A cyclic process of heat transfer for effecting separation of waterfrom an aqueous solution of a nonvolatile solute (SW), which comprisesthe following steps:

a. effecting heat transfer between said solution (SW) and the condensingvapor (IV) of water immiscible liquid (IL) which has a lower boilingpoint than water and is hotter than the solution (SW) to transfer latentheat of condensation of the vapor (IV) to the solution (SW) to therebyheat the solution (SW) and condense the vapor (IV), such heat transferbeing effected by at least one of the following methods: (I) directcontact of the vapor (IV) with the solution (SW) and (2) direct contactof the vapor (IV) with its condensate (IL) and thence by direct contactof the condensate (IL) with the solution (SW);

b. separating heated solution (SW) produced in step (a) from the contactmixture of solution (SW) and condensate (IL) of immiscible vapor (IV)and flash evaporating the separated heated solution (SW) produced instep (a) c. condensing water vapor (WV) from step (b) by direct contactwith cooler fresh water (FW) withdrawing as product part of the freshwater (FW) produced in step (c) e. effecting direct contact between (1)another part of the fresh water (FW) produced in step (c) and (2) condensate (IL) from step (a) to vaporize said condensate (IL) and cool thewater, 1 I

f. recycling vapor (IV) from step (e) to step (a) and recycling coldwater (FW) from step (e) to step (c) g. withdrawing from the process atleast a portion of the concentrated solution (SW) produced in step (b)and h. providing heat input to the system to separate solvent fromsolute and to make up for heat losses.

2. The process of claim 1 wherein the immiscible liquid is a hydrocarbonliquid (IL).

3. The process of claim 2 wherein the hydrocarbon liquid has two or moremolecular species and has a substantial boiling range.

4. The process of claim 2 wherein the hydrocarbon liquid has a narrowboiling range.

5. The process of claim 2 wherein the aqueous solution is sea water.

6. A process according to claim 1 wherein heat is introduced into thesystem by at least one of the following methods: (1) compressing thevapor (IV) of immiscible liquid (IL); (2) compressing the vapor of water(WV); (3) direct injection of steam; and by (4) indirect heat exchangebetween a fluid circulating in the system and a heated fluid external tothe system.

7. A process according to claim 6 wherein a portion of heat input to thesystem is by compression of at least one of the vapors (IV) and (WV),another portion of the heat input is by at least one of the methodsconsisting of direct injection of steam and indirect heat exchange ofsteam with a circulating circulating liquid phase, a gas turbine isemployed to operate a compressor to provide said compression and wasteheat from the gas turbine is used to produce steam which is used for theaforesaid steam heat input.

8. A process of producing fresh water from saline water which comprisesthe following steps:

a. providing a first zone for heat exchange between the vapor (IV) of awater immiscible liquid (IL) and saline water (SW), a second zone forheat exchange between fresh water (FW) and water immiscible liquid (IL),a third zone for flash evaporation of saline water (SW) and a fourthzone for condensation of vapor (WV) of fresh water (FW),

. introducing into said first zone saline water (SW) and the vapor (IV)of a water immiscible liquid (IL), said vapor (IV) being hotter than thesaline water (SW said immis: cible liquid (IL) having a lower boilingpoint than water, being less dense than water and having a substantialboiling range c. causing countercurrent flow of said vapor (IV) and saidsaline water (SW) in the first zone and causing condensation of thevapor (IV) and resulting heating of the water (SW) therein by at leastone of the following means: (I) direct contact between vapor (IV) andsaline water (SW) and (2) direct contact between vapor (IV) and itscondensate (IL) and transfer of heat by direct contact of condensate(IL) and saline water (SW),

d separating and withdrawing from the first zone immiscible liquid (IL)resulting from condensation of vapor (IV) and also withdrawing heatedsaline water (SW),

e. introducing the heated saline water (SW) withdrawn from the firstzone into the third zone and introducing immiscible liquid (IL)withdrawn from the first zone into the second zone, 1

f. flash vaporizing heated saline water (SW) in the third zone andintroducing the resulting water vapor (WV) into the fourth zone.

g. recycling unevaporated saline water (SW) from the third zone to thefirst zone,

h. condensing water vapor (WV) in the fourth zone by direct contact withcolder fresh water (FW i. withdrawing fresh water (FW), including watervapor condensate produced therein from the fourth zone and introducingit into the second zone,

j. effecting countercurrent flow and contact of immiscible liquid (IL)and fresh water (FW) in the second zone, thereby vaporizing immiscibleliquid (IL) and cooling fresh water (FW),

k. withdrawing cooled fresh water (FW) from the second zone andintroducing it into the fourth zone to condense water vapor (WV) thereinand withdrawing vapor (IV) of immiscible liquid (IL) from the secondzone and introducing it into the first zone,

I. withdrawing as product a portion of the fresh water (FW) circulatingbetween the second and fourth zones and withdrawing a portion of thesaline water (SW) circulating between the third and first zones, and

m. providing heat input to the system to separate solvent from soluteand to make up for heat losses.

9. The method of claim 8 wherein the hot liquid saline water (SW)introduced into the third zone and the cool liquid fresh water (FW)introduced into the fourth zone are caused to flow in such manner that,as water vapor (WV) is distilled from each portion of the third zone itcondenses in a portion of the fourth zone wherein the temperature of theliquid fresh water (FW) is only slightly lower than the temperature ofthe saline water (SW) in the aforesaid portion of the third zone.

10. The process of claim 8 wherein the immiscible liquid is ahalocarbon.

11. The method of claim a wherein a substantial depth of immiscibleliquid (IL) is maintained above the liquid aqueous phase in the firstand second zones to seal the latter from the vapor phase.

12. The method of claim 11 wherein agitation is provided to promotecontact of the liquid phases with one another but to minimize contact ofthe liquid aqueous phases with the vapor phases.

2. The process of claim 1 wherein the immiscible liquid is a hydrocarbonliquid (IL).
 3. The process of claim 2 wherein the hydrocarbon liquidhas two or more molecular species and has a substantial boiling range.4. The process of claim 2 wherein the hydrocarbon liquid has a narrowboiling range.
 5. The process of claim 2 wherein the aqueous solution issea water.
 6. A process according to claim 1 wherein heat is introducedinto the system by at least one of the following methods: (1)compressing the vapor (IV) of immiscible liquid (IL); (2) compressingthe vapor of water (WV); (3) direct injection of steam; and by (4)indirect heat exchange between a fluid circulating in the system and aheated fluid external to the system.
 7. A process according to claim 6wherein a portion of heat input to the system is by compression of atleast one of the vapors (IV) and (WV), another portion of the heat inputis by at least one of the methods consisting of direct injection ofsteam and indirect heat exchange of steam with a circulating circulatingliquid phase, a gas turbine is employed to operate a compressor toprovide said compression and waste heat from the gas turbine is used toproduce steam which is used for the aforesaid steam heat input.
 8. Aprocess of producing fresh water from saline water which comprises thefollowing steps: a. providing a first zone for heat exchange between thevapor (IV) of a water immiscible liquid (IL) and saline water (SW), asecond zone for heat exchange between fresh water (FW) and waterimmiscible liquid (IL), a third zone for flash evaporation of salinewater (SW) and a fourth zone for condensation of vapor (WV) of freshwater (FW), b. introducing into said first zone saline water (SW) andthe vapor (IV) of a water immiscible liquid (IL), said vapor (IV) beinghotter than the saline water (SW), said immiscible liquid (IL) having alower boiling point than water, being less dense than water and having asubstantial boiling range c. causing countercurrent flow of said vapor(IV) and said saline water (SW) in the first zone and causingcondensation of the vapor (IV) and resulting heating of the water (SW)therein by at least one of the following means: (1) direct contactbetween vapor (IV) and saline water (SW) and (2) direct contact betweenvapor (IV) and its condensate (IL) and transfer of heat by directcontact of condensate (IL) and saline water (SW), d. separating andwithdrawing from the first zone immiscible liquid (IL) resulting fromcondensation of vapor (IV) and also withdrawing heated saline water(SW), e. introducing the heated saline water (SW) withdrawn from thefirst zone into the third zone and introducing immiscible liquid (IL)withdrawn from the first zone into the second zone, f. flash vaporizingheated saline water (SW) in the third zone and introducing the resultingwater vapor (WV) into the fourth zone. g. recycling unevaporated salinewater (SW) from the third zone to the first zone, h. condensing watervapor (WV) in the fourth zone by direct contact with colder fresh water(FW), i. withdrawing fresh water (FW), including water vapor condensateproduced therein from the fourth zone and introducing it into the secondzone, j. effecting countercurrent flow and contact of immiscible liquid(IL) and fresh water (FW) in the second zone, thereby vaporizingimmiscible liquid (IL) and cooling fresh water (FW), k. withdrawingcooled fresh water (FW) from the second zone and introducing it into thefourth zone to condense water vapor (WV) therein and withdrawing vapor(IV) of immiscible liquid (IL) from the second zone and introducing itinto the first zone, l. withdrawing as product a portion of the freshwater (FW) circulating between the second and fourth zones andwithdrawing a portion of the saline water (SW) circulating between thethird and first zones, and m. providing heat input to the system toseparate solvent from solute and to make up for heat losses.
 9. Themethod of claim 8 wherein the hot liquid saline water (SW) introducedinto the third zone and the cool liquid fresh water (FW) introduced intothe fourth zone are caused to flow in such manner that, as water vapor(WV) is distilled from each portion of the third zone it condenses in aportion of the fourth zone wherein the temperature of the liquid freshwater (FW) is only slightly lower than the temperature of the salinewater (SW) in the aforesaid portion of the third zone.
 10. The processof claim 8 wherein the immiscible liquid is a halocarbon.
 11. The methodof claim a wherein a substantial depth of immiscible liquid (IL) ismaintained above the liquid aqueous phase in the first and second zonesto seal the latter from the vapor phase.
 12. The method of claim 11wherein agitation is provided to promote contact of the liquid phaseswith one another but to minimize contact of the liquid aqueous phaseswith the vapor phases.